The effect of hemorrhagic hypotension on plasma serotonin levels has been investigated previously,1,2 but it remains to be shown conclusively whether or not the level of this biogenic amine rises, falls, or remains constant as a result of hypotension. Since serotonin (5-HT) is rapidly metabolized to 5-hydroxyindole acetic acid (5-HIAA), changes in plasma 5-HT are reflected by changes in the plasma 5-HIAA levels.3,4 In the present study, simultaneous determinations of plasma 5-HT and of plasma 5-HIAA, the predominant end-product of 5-HT metabolism, are made at specific intervals before and during hypotension induced by hemorrhage in an attempt to contribute further information on this subject. Methods and Materials Reagent grade chemicals and solvents were used in the serotonin determinations made by a method modified from Waalkes.5 These included the following: (1) NaCl; (2) n-butyl alcohol; (3) n-heptane (commercial n-heptane washed with equal volumes of 0.1